According to La Bible des Noëls Anciens et nouveaux (1790), pp. 77-79, Bel Astre Que J'Adore is sung to the tune "Charmante Gabrielle", purported to have been written by Henri IV. Per online sample pages of a modern book, it also appears in J.B. Weckerlin's 3-volume collection Echos du Temps Passé, though I haven't tried to hunt this up to see if a tune is given.

Claude Nadeau generously provides a free MP3 of an organ (and oboe) rendition of two Breton noëls: "Bel Astre" and "Pe trouz war an douar" (page) (sound clip. The oboe is played by Allison Smith.
Her page also provides words for both, and a translation of the latter from Breton to French. She says that "Bel Astre" is from the region of Guérande.